Simplify the following expression: $k = \dfrac{15r - 30s}{10t + 25r} - \dfrac{10s + 25r}{10t + 25r}$ You can assume $r,s,t \neq 0$.
Answer: Since the expressions have the same denominator we simply combine the numerators: $k = \dfrac{15r - 30s - (10s + 25r)}{10t + 25r}$ $k = \dfrac{-10r - 40s}{10t + 25r}$ The numerator and denominator have a common factor of $5$, so we can simplify $k = \dfrac{-2r - 8s}{2t + 5r}$